cBN is the hardest material next to diamond, and a sintered compact of cBN is employed for various cutting tools.
As one example of cBN sintered compacts suitable for cutting tools, Japanese Patent Publication No. 57-3631 (corresponding U.S. Pat. No. 4,334,928) discloses a hard sintered compact for tools, containing 80-40% by vol. of cBN and the remaining proportion of carbide, nitride, boride and silicide of elements selected from groups of the IVa, Va and VIa periodic table, mixtures thereof or counter solid solution compounds as principal components. These compounds constitute a continuous bound phase in the texture of the sintered compact. This sintered compact exhibits high performance in general as a material for cutting tools; however, it has a disadvantage that the cutting edge of the cutting tool is liable to be damaged due to insufficient strength and abrasion of the cutting edge when subjected to a considerably strong impact, for example, in the application for a continuous cutting of a highly hardened steel.
An improved hard sintered compact for tools, in which the strength and wear resistance of a cutting edge made of the sintered compact is improved in order to eliminate the damage of the cutting edge, is disclosed in Japanese Patent Laying-Open No. 62-228450. In this sintered compact, a binder includes 25-50% by wt. of Al, a compound containing Ti such as carbide of Ti, and 4-40% by wt. of W contained in the compound containing Ti or contained as WC. These components react with cBN in sintering, to produce aluminum boride, titanium boride and the like, which serve to firmly bond the binder and cBN or another binder.
U.S. Pat. No. 4,911,756 discloses a hard sintered compact for tools, including 50-75% by vol. of cBN and 25-50% by vol. of a binder containing 20-50% by wt. of aluminum, carbon nitride titanium and the like and 4-40% by wt. of tungsten.
Even the sintered compacts disclosed in the above-described Japanese Patent Publication No. 57-3631, Japanese Patent Laying-Open No. 62-228450 and U.S. Pat. No. 4,911,756 have, however, the following disadvantages when used for tools for cutting a cast iron. In cutting of a graphite cast iron and fast cutting of a gray cast iron, for example, abrasion of the cutting edge abruptly proceeds, so that the life of the cutting edge is shortened. Further, crater abrasion occurs in the cutting edge, so that the cutting edge is damaged. These problems still remain unsolved.